To assist in selecting the optimal vector system for use in the CHAVI clinical studies, a comparison was done in rhesus monkeys between VSV and Ad to assess the magnitude and durability of vaccine-elicited T lymphocyte immune responses. The VSV vectors used in this study were made with N4CT9, a pathologically attenuated strain of the vims developed by Wyeth. The immunogenicity of these vectors was compared with that of an Eldeleted, E3-inactivated Ad5 developed by Gary Nabel, VRC and GenVec. Both vectors carried an identical HIV-1 HXBc2 gag gene insert. Ad5-seronegative monkeys were immunized at 0 and 8 weeks. Six monkeys received intramuscular inoculations of 108 pfu rVSV Indiana followed by the same dose of rVSV New Jersey, a vector that is not cross-neutralized by antibodies generated by rVSV Indiana. Six monkeys received the same immunogens delivered by the intratracheal route. Another 6 monkeys received 1011 rAd5 followed by the same dose of the same recombinant vector. Cellular immune responses were monitored by pooled peptide Elispot and ICS assays. As shown below, significant Gag-specific immune responses were only detected in the rVSVimmunized monkeys following the boosting immunization. Moreover, the immune responses elicited by the rVSV following the boosting immunization were comparable in magnitude following intratracheal and intramuscular administration, and reached the same magnitude seen following a single immunization with the rAd5 vector.